This invention relates to a manual or automatic microwave tuner to be used mainly in wideband automatic load pull testing of power transistors or noise testing of low noise transistors by being able to synthesize the amplitude and phase of selected impedances.
Modern design of high power microwave amplifiers and oscillators, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is necessary for the transistors to operate in their highly non-linear region, close to power saturation, and it is inadequate to be described as such using non-linear analytical or numerical models, in order to design power amplifier circuits.
A popular method for testing and characterizing microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance (or reflection factor) conditions under which the Device Under Test (DUT, or transistor) is tested (FIG. 1).
When transistors operate in the non-linear range at high power and close to saturation, signal distortion inside the transistors creates significant harmonic frequency signals that reduce the efficiency and the signal transmission purity of the communication system. In order to improve and optimize the performance of such transistors under these operating conditions, the tuners used to test those high power DUT must provide for very high reflection factors of typically 0.95 to 0.98, corresponding to very low internal impedances of the DUT of the order of 0.5 Ohm or less.
The tuners used in such automatic load pull set-ups must be able to be pre-characterized (calibrated) for different states and at desired frequencies, using a calibrated Vector Network Analyzer (VNA) and to be able to reproduce repeatably the calibrated states.
There are essentially two types of tuners which can be used to allow generation and manipulation of microwave impedances presented to the DUT:
A. Passive electro-mechanical [7] or passive electronic tuners [6], leading to “passive load pull” (FIGS. 1, 2); and;
B. Active tuners, leading to “active load pull” [8].
Electro-mechanical tuners [7] have a number of advantages compared to active tuners [8], such as long-term stability, higher handling of microwave power, much easier operation and lower cost.
Passive electronic tuners [7] have been used in the past, but they provide limited tuning range and handling power and do not offer any significant benefit within the scope of this invention.